Battery operated electronic devices, such as lap-top computers, cellular telephones, calculators, etc., require conversion from the voltage output by the batteries powering the devices to a voltage required by the circuitry of the devices. Typically, a rechargeable battery produces a 1.5v voltage. In some applications, several batteries are connected in series, thereby increasing the voltage output from 1.5 to higher voltages such as 6 to 12 volts. However, most integrated circuits, which make-up large portions of battery operated electronic devices, require a voltage supply of either 5.0 volts or 3.3 volts. Thus, the voltage produced by the battery must be converted to the voltage requirements of the device circuitry.
Switching regulators provide one mode of dc-dc power conversion. FIG. 2(a) is a simplified schematic showing a switching regulator. A switching regulator may be viewed as an inverter circuit connected to a low-pass filter composed of an inductor and a capacitor. The inverter circuit produces a square wave voltage waveform having on-time during pulses and off-time between pulses. The low-pass filter smoothes the waveform, thereby producing a nearly constant level dc-voltage. During on-time the capacitor charges and it discharges during off-time. The voltage level is regulated in the switching regulator by controlling the duration and frequency of the voltage pulses produced by the switching network, i.e., by controlling the on-time versus the off-time. The ratio of on-time versus the total time for both on-time and off-time is referred to as the duty cycle. By lowering the duty cycle the voltage is lowered because the charge-up time for the capacitor is shortened and the discharge-time is lengthened. Conversely, by increasing the duty cycle the voltage is increased because the charge-up time is lengthened and the discharge-time is shortened.
One form of switching regulator uses transistors as the switches in the inverter network. The switches are turned on and off by providing a current or a voltage to the transistors'gates. The frequency with which the transistor is turned on and off is controlled by a pulse-width modulator (PWM). Such controllers are referred to as PWM controllers. An example of a PWM controller is the LM1575 family of voltage regulators available from National Semiconductor Corporation of Santa Clara, Calif.
The value of a portable device is proportional to the duration of the device's battery life. Thus, it is very important to minimize the power consumption of the device. A number of techniques have been implemented to reduce the power consumption and, thereby, prolonging the battery life of portable devices. For portable computers, such techniques include shutting down the display and/or the micro-processor during idle times. While the display and micro-processor are significant sources of power consumption, the power supply also consumes a considerable amount of power.
In a switching regulator, during low loads, the power consumption may be reduced by decreasing the duty cycle. However, there is still a relatively high power consumption in the switching network due to the capacitance in the transistors and to bias current. Bias current is the current required to keep the circuitry active. The bias current is the current that is required by the transistor inverter network even when the circuit is not switching.
U.S. Pat. No. 5,028,861 to Pace et al. describes one method of reducing the power consumption due to a dc-dc converter power supply. This method consists of a regulating means whereby the power conversion is turned on and off depending on the output voltage. When the output voltage has increased above a predetermined threshold, the converter is shut off, thereby, allowing the capacitor to discharge to a lower limit. At this lower limit, the power conversion is reactivated, thereby allowing the capacitor to charge. The alteration between turning the power conversion on and off causes a relatively large ripple voltage during high current load as well as during low current load. Having a large ripple voltage is disadvantageous because it places a larger filtering burden on the functional circuitry of the device.
Furthermore, during low loads, the technique described in U.S. Pat. No. 5,028,861 is relatively inefficient because, even during low loads, the power conversion is turned on and off, thus requiring voltage to be applied to the gating transistors and to the bias circuitry.
Furthermore, the output voltage waveform produced by the controller described in U.S. Pat. No. 5,028,861 is of variable frequency at all times.
Another power saving technique is exemplified by the LTC1148 family available from switching-regulator controllers of Linear Technology Corp., of Milpitas, Calif. During high current load, the LTC1148 exhibits the behavior of a PWM controller. During low loads, the LTC1148 allows the output voltage to drift out of regulation towards an upper limit. At this stage, the LTC1148 resembles the controller described in U.S. Pat. No. 5,028,861, in that, at the upper limit, the power conversion is shut-down and the capacitor is allowed to discharge until the output voltage has dropped to the lower limit. When the output voltage has dropped below the lower limit, the LTC1148 returns to PWM mode, i.e., it controls the power conversion by turning on and off the voltage to the gates of the switching transistors. Thus, the LTC1148 is inefficient during low current load operations because it turns on and off the gate voltage of the switching transistors.
The LTC1148 relies on an external sensing resistor for sensing current flow through the switching circuitry. The resistor is a further cause of power consumption during both low and high loads. Additionally, when the input voltage changes, the LTC1148 changes the switching frequency, which places a larger noise filtering burden on the circuitry.
It is therefore desirable to provide a system and method for dc-dc power conversion that conserves power during low loads and yet overcomes the problems described above with reference to the prior art.